Brief Description Of The Prior Art
Large tanks are currently provided for storing and dispensing liquids, such as fracturing fluids, drilling muds, crude oil and various other liquids or slurries at a time when the tank is stationary, but which, when the tank has been emptied, are susceptible to over-the-road towing. These tanks are generally of relatively large capacity, holding thousands of gallons of liquid. They typically have lengths of 30-40 feet and a height of from about 9 feet to 12 feet.
In general, tanks constructed for the described purpose have ground-engaging skids which permit them to be towed short distances to a situs of use, or moved, even when full, from one location to the other by sliding on the skids. For over-the-road transport, however, such tanks generally have a king pin at the forward end thereof, and are moved by engaging the king pin with the fifth wheel of a tractor vehicle. The tractor vehicle then provides the motive power for moving the empty tank a substantial distance in over-the-road transport. The fifth wheel of the tractor vehicle is frequently located at a vertical level which is from about 3 to 5 feet above the surface of the roadway. The front end of the storage tank is therefore elevated by that amount when the king pin is engaged with the fifth wheel for towing, because the fifth wheel is located beneath the front end of the tank when the tank is in the towing status. This elevation creates a concern for maintaining the maximum height of the tank at a level which is below the clearance afforded beneath underpasses, and, in most instances, in order also to comply with legally established maximum height dimensions. Because of this consideration, the geometry of the tank becomes of concern when attempting to stay within the described maximum height constraints, and yet provide a sufficiently large volume in the tank to accommodate the liquid loading which is desirable. In order to accomplish this, tanks have been made in various configurations, such as trapezoidal or with a stepped top or roof of incrementally decreasing height, so as to afford more internal volume in the same length and width of tank, and yet avoid the forward end of the tank extending to a height such that the tank exceeds the maximum legal height when transported, or a risk of collision with underpasses or the like is possible.
Another consideration which has entered prominently into the design of large liquid storage tanks of the type described is the need to have ready and unobstructed access to the interior of the tank from the upper side thereof. From a work station at this location, the maximum level of liquid in the tank can be determined and controlled, the liquid contents can be gauged with simple instruments, the tank can be cleaned and other necessary operations can be performed with maximum facility and ease.
In some types of tanks of this sort, in order to provide the access and work area which has been described, a flat surface has been provided on the tank roof near the rear of the tank. From this location, access is had to the interior of the tank for performing various operations at the time when the tank is in use for the storage of liquid, or is being placed in use or is being decommissioned. In such instances, particularly with the trapezoidal tanks which have been described, the relatively smaller forward end of the tank extends to a substantially lower height at a time when the bottom of the tank rest flatly on the ground (as contrasted with being hoisted up for king pin/fifth wheel engagement for over-the-road transport). This necessitates movement by personnel from the forward end to the rear end of the tank by walking up the inclined roof surface from the forward end of the tank toward the flat work area at the rear of the tank roof. Such access entails danger to working personnel when the tank is wet or slippery due to rain or ice or snow accumulation on the tank roof.